1. Field of the Invention
This invention relates to a novel process for treating lignocellulose in a confined chamber in the absence of added oxygen with an intially neutral or acidic solvent mixture comprised of water and a lower aliphatic alcohol having one to three carbon atoms, a dissolved magnesium, calcium or barium salt primary catalyst preferably augmented by a very minor amount of an acidic compound as a secondary catalyst by cooking at a temperature in the range 145.degree. C. to about 240.degree. C., to produce high yields of chemical pulp of strong separated cellulose fibers.
The process is particularly successful in producing high yields of pulp of separated fibers even with residual lignin contents exceeding 80 Kappa number without requiring mechanical refining to liberate fibers. Such pulps have nearly theoretical alpha-cellulose content and fiber strength only slightly below the strength of natural undegraded cellulose. The process is universally effective in treating the gymnosperm and angiosperm wood species as well as lignocellulosic plant materials such as bamboo, sugarcane, cereal plants and grasses.
2. Description of the Prior Art
The objectives in an ideal process for cooking lignocellulose are met when virtually all the lignin becomes solubilized in a short cooking time, with only an absolute minimum of other cell wall materials encrusting the cellulose fibers, while fiber yields almost equal to the total content of cellulose and hemicellulose are attained. Such efficient cooking would minimize the energy required in mechanical dispersion of the fibers after cooking and also minimize bleach chemical consumption.
For complete delignification the solubilization must proceed within the cell structure, not only to the fiber-cementing layers or middle lamella composed of lignincarbohydrate matrix, but also to the cell wall matrices containing varying proportions of lignin and hemicelluloses. When virtually complete delignification of these structures has been reached the proportion of screened rejects will be very low and the cooked chips will require little if any mechanical agitation for full defiberization, saving on costs of process energy and preserving good fiber properties.
The prior art includes processes wherein wood is subjected to rapid hydrolysis in aqueous or aqueous-organic solvent mixtures containing acid and/or acidic salt catalyst compounds at temperatures in the range 100.degree. C. to about 230.degree. C. in a confined chamber in absence of added oxygen. The most important process is disclosed in United Kingdom Patent No. 357,821 to Kleinert and Tayenthal (1932), and includes an alcohol-water mixture containing slight quantities of inorganic or organic acid, or an acidic salt such as sodium bisulphite or sodium bi-sulphate.
The present invention is not related to basic hydrolysis of lignocellulosic materials which is also described by Kleinert et al which is a different process chemically. Basic hydrolytic agents such as alkali metal and alkaline earth metal oxides or hydroxides or basic salts such as sodium carbonate or magnesium carbonate are also described. The magnesium carbonate is used in the solvent mixture in the same manner as the oxides or hydroxides and is quite insoluble in alcohol and water at room temperatures thus providing few magnesium ions in solution. Magnesium carbonate was not used under acidic conditions by Kleinert et al.
In U.S. Pat. No. 2,951,775 to Apel it is proposed to hydrolyze wood with a lower aliphatic alcohol and a large proportion of hydrochloric acid. Saccharification is taught also by U.S. Pat. No. 2,959,500 to Schlapfer and Silberman using ethanol or n-propanol and water containing strong acid between 0.0125 N and 0.15 N at 170.degree. C. to 180.degree. C., who also disclose the use of ferrous ammonium sulphate as salt catalyst with sulphuric acid. At column 4, lines 17 to 35 and Examples 1 and 2 the use of metal salts generally is disclosed to be disadvantageous to the organosolv and hydrolysis process. Recovery of cellulose and of lignin from lignocellulose is proposed in U.S. Pat. No. 2,308,564 to McKee by cooking in water carrying a high concentration of alkali metal xylene sulphonate. U.S. Pat. No. 2,022,654 to Dreyfus describes a basic solvent process as does U.S. Pat. No. 2,022,664 to Groombridge et al.
U.S. Pat. No. 3,701,712 to Samuelson et al, U.S. Pat. No. 3,725,194 to Smith and U.S. Pat. No. 3,652,385 to Noreus describe an alkaline aqueous mixture for cellulose separation in the presence of added oxygen for delignification. Catalysts including magnesium, calcium and barium salts are used in the process. These basic processes vigorously attack the lignins so that they are severely degraded. These processes are different from the non-oxidation processes of Kleinert et al and require more elaborate processing equipment.
The present invention is in the same field as our copending U.S. application Ser. No. 248,023, filed Mar. 26, 1981, now U.S. Pat. No. 4,409,032, wherein processes are described for cooking with alcohol-water mixtures containing a selected organic acid or buffered inorganic acid, to produce pulps in very short times and to recover high quality soluble lignin and sugars.
In such acid-catalyzed organosolv processes (as well as the basic oxidation processes) although the lignin and sugar products are of considerable value, a major disadvantage from the standpoint of pulp acceptability for making paper is that the cellulose fibers are attacked throughout the cooking interval so that before an acceptably low residual lignin is reached, degradation of the cellulose chains will have occurred. The viscosity number of the cellulose will be much below that of the natural undegraded cellulose, so that paper sheets made from the pulps lack high breaking strength, tear and burst strength desirable for industrial paper products. Some degradation of lignin by acid-catalyzed recondensation and some conversion of sugars to dehydration products also occurs.
A further disadvantage of earlier alcohol-water cooks as exemplified in U.S. Pat. No. 3,585,104 to Kleinert and in U.S. Pat. No. 4,100,016 to Diebold et al is the poor solubility in the cooking solvent mixture of lignin which has become partially recondensed, causing blockage of micropores of the wood. Not only is severe undercooking of chip cores likely, but gummy deposits tend to form in pipes and cooking vessels when the cooking liquor is allowed to cool substantially below the cooking temperature.